1. Field of the Invention
The present invention relates to a medium holding apparatus and an image forming apparatus, and more particularly to a medium holding apparatus suitable for holding and conveying paper in an image forming apparatus such as an inkjet recording apparatus, and an image forming apparatus in which such a medium holding apparatus is employed.
2. Description of the Related Art
As a general image forming apparatus, there is an inkjet recording apparatus, which forms a desired image on a recording medium by ejecting and depositing a plurality of colors of inks onto the recording medium from a plurality of nozzles provided in an inkjet head. The recording media used in the inkjet recording apparatus are not just paper media, but also include media of a plurality of types, such as resin sheet, metal sheet, and the like, and furthermore media of various sizes and thicknesses are used.
A conveyance member which holds and conveys the recording medium has a drum shape or belt shape, or the like. For the method of holding the recording medium, it is suitable to use an air suction method which holds the recording medium by applying a suction pressure (negative pressure) to the recording medium from inside the conveyance member through suction apertures arranged in the surface of the conveyance member.
In the air suction method described above, if the suction pressure is insufficient, then there is a possibility of positional displacement of the recording medium, and if the suction pressure is excessive, then there is a possibility of deformation of the recording medium, or the occurrence of image abnormalities caused by the ink droplets which have been deposited on the recording medium being sucked into the recording medium due to the suction pressure, or the like. Furthermore, if a plurality of suction apertures are provided in accordance with the maximum size so as to achieve compatibility with a plurality of media sizes, and the plurality of suction apertures are suctioned by a common pump, then if there are open suction apertures in cases where a recording medium of small size is used, air might leak through the open suction apertures giving rise to defective holding of the recording medium due to insufficient suction pressure. Consequently, various ways have been devised in order to avoid problems of these kinds.
Japanese Patent Application Publication No. 11-240133 discloses controlling a pressure drum of a printer so that a suction pressure is applied only to suction elements in a range where paper is present.
Japanese Patent Application Publication No. 9-123395 discloses a printer which is made to correspond to different paper sizes by exchanging a porous sheet.
However, a method of holding and securing the recording medium by air suction involves a complex mechanism in order to achieve a high suction pressure. Furthermore, in the related art structure, since the same suction pressure acts on the whole area of the paper, it is necessary to employ a larger suction flow volume in order to secure thick paper, or “stiff” paper. In particular, a strong suction pressure is required in the trailing end portion of the recording medium.